1. Field of the Invention
The present invention relates to an apparatus for polishing workpieces such as semiconductor wafers, various kinds of hard disks, glass substrates and liquid crystal display panels.
2. Description of the Related Art
In a conventional chemical mechanical polishing (CMP) apparatus used in fabrication of a semiconductor integrated circuit, a semiconductor wafer is held by a holder called a "top ring" and is rotated and pressed against a polishing cloth mounted on a rotating turntable while being supplied with abrading slurry including free abrading grains at a sliding interface. However, such a CMP apparatus presents a problem that, depending on the type of surface patterns and differences in the heights of fine surface structures fabricated on the wafer, it is not possible to obtain a precisely polished flat surface.
Therefore, in place of the above-mentioned CMP process, another CMP technique has been developed, where the wafer is placed in sliding contact with a solid polishing member shaped usually in the form of a plate, in which abrading grains are bound in a matrix, while a polishing liquid or a polishing solution is supplied at the sliding interface. The solid polishing members include variations such as a ring-type member or a cup-type member having abrading pellets distributed in a ring shape.
FIG. 11 illustrates basic movements of a cup-type polishing member. A cup-type polishing member 80 has a ring-shaped abrading member 81 attached on the bottom surface of a polishing member holder 83, and is pressed against a wafer 100 held in a wafer holder 85. Both are rotated, for example, in the same G, H directions, and the wafer 100 is uniformly polished by moving the polishing member 80 linearly in the radial direction of the wafer 100 (indicated by the arrow I) so that the abrading member 81 polishes entire surface of the wafer 100. The polishing member holder 83 is connected to the drive shaft 89 through a spherical bearing 87 so as to transmit a pressing force F from the drive shaft 89 through the spherical bearing 87, and coupling of drive pin 91 passive pin 93 transmits the rotation H from the drive shaft 89.
In general, the polishing member 80 is pressed on the wafer 100 through the drive shaft 89, therefore, when drive axis k of the drive shaft 89 is projected within the wafer 100, as shown in FIG. 11, there is no tilting of the polishing member 80. But, when it is in the position shown in FIG. 12, the rotation axis k projects outside the wafer 100, and even if a part of the abrading member 81 is on the wafer, lever action produces tilting of the abrading member 81 about a fulcrum at the edge of the wafer 100. This prevents the abrading member 81 from having a planar contact with the wafer 100, and polishing becomes impossible. Therefore, to avoid such a situation, conventional abrading member 81 could only move within an area of support for the drive axis k. This problem is the same in a conventional polishing apparatus using a top ring holding the wafer to press it against a polishing table.